1. Field of the Invention
The present invention relates to a magnetic recording medium such as magnetic recording drums, magnetic recording tapes, magnetic recording disks, magnetic recording cards, and so on, and a magnetic storage apparatus. More particularly, the invention relates to a perpendicular magnetic recording medium suitable for ultra-high density magnetic recording of 50 gigabits or more per 1 square inch, and a magnetic storage apparatus using the same.
2. Description of the Related Art
With the popularized use of the Internet in recent years, there have been increases not only in the shipping number of personal computers, but also in the demand for a magnetic recording disk device. Connection to the Internet can be made not only by a personal computer but also by a portable terminal. However, to make the portable terminal itself more convenient, it is essential to incorporate the magnetic recording disk device, and the demand in this field is also expected to grow in the future. In addition, as digital television broadcasting is near at hand, the use of the magnetic recording disk device as a video recorder has begun on a full scale. Accordingly, the application field of the magnetic recording disk device has been increasingly expanded. Still, however, further miniaturization and a larger capacity have been requested by users.
The magnetic recording disk device thus far available has employed an in-plane recording system. In in-plane recording, the recording direction of magnetization is in-plane, and adjacent magnetization is reverse in polarity. Thus, between adjacent recording bits, a magnetization transition region is formed in order to reduce magnetostatic energy. A large width of the magnetization transition region causes an increase in noise and, thus, to reduce noise, thin formation of a magnetic layer and micronization of the magnetic crystal grain size are considered to be effective. Therefore, an approach to the high recording density of an in-plane recording medium is to focus on how to reduce the volume of a very small magnet constituting a recording bit. It is generally considered, however, that it will be difficult to deal with a much higher recording density expected in the future, with the in-plane recording medium, because of a physical limitation. In other words, the in-plane recording medium may have a problem in basic performance for saving recording information, due to the thermal fluctuation phenomenon of magnetization following the micronization of the very small magnet constituting the recording bit.
For the foregoing reason, a perpendicular magnetic recording medium has again attracted attention recently. In perpendicular recording, the direction of recorded magnetization is perpendicular to a film plane, and no strong charge is present between adjacent recording bits, preventing the width of a magnetization transition region from becoming as large as that of the in-plane medium. Thus, with a much higher density of the magnetic recording disk device in mind, high expectation is now placed on the potential of a perpendicular magnetic recording system.
With regard to the perpendicular magnetic recording medium, there are largely two, i.e., a single-layered perpendicular medium and a double-layered perpendicular medium. The double-layered perpendicular medium includes a soft magnetic layer between a magnetic layer and a substrate for saving information, and the presence of this soft magnetic layer is a main difference from the single-layered perpendicular medium. Each medium has advantages and disadvantages. At present, however, the combined system of the double-layered perpendicular medium with a single magnetic pole head is most promising.
As a practical problem, also in the perpendicular magnetic recording medium, there is a problem of a reduction in read output, caused by the thermal fluctuation of magnetization. To solve this problem, it is important to enhance perpendicular orientation of an easy magnetization axis (c axis) regarding the magnetic layer mainly containing Co having a hexagonal close-packed structure (h. c. p.).
With regard to the conventional perpendicular medium, a technology has been proposed to provide a non-magnetic underlayer having an h. c. p. structure between the substrate and the magnetic layer, to enhance perpendicular orientation. An example is a TiCr underlayer (Journal of Magnetism and Magnetic Materials, 193, pp. 253-257 (1999)). A main element of the TiCr underlayer is Ti, obtained by adding 20 at. % or lower of Cr. A crystal is grown in such a way as to set the c axis of the underlayer perpendicular to the film surface, and by growing a magnetic layer thereon in a heteroepitaxial manner, the perpendicular orientation of the easy magnetization axis of the magnetic layer can be enhanced. Other than the non-magnetic h. c. p. underlayer, a technology for providing an underlayer containing Pt has been proposed (Journal of Magnetics Society of Japan, 24, pp. 267-270 (2000)).
To counter the thermal fluctuation of the perpendicular magnetic recording medium, it is important not only to enhance the perpendicular orientation of the axis of the easy magnetization, but also to increase squareness at least to 0.9 or more simultaneously. There are two methods of obtaining squareness, i.e., one by VSM measurement based on Mr/Ms in M-H loop, and the other by Kerr effect measurement. Regarding the foregoing TiCr underlayer, one satisfactory to a certain level in terms of perpendicular orientation can be obtained. Because of small squareness, however, the influence of thermal fluctuation is large, causing a reduction in read output. On the other hand, regarding the Pt underlayer, there is a difference in size from the crystal lattice of the magnetic layer though (111) orientation of the Pt underlayer is strong. Consequently, lattice matching is bad, making it difficult to improve perpendicular orientation as expected.
Further, to achieve a high recording density for the perpendicular magnetic recording medium, a reduction in medium noise becomes an important technical subject. An effective way of achieving low noise is to reduce the crystal grain size of the magnetic layer. Because of the heteroepitaxial growth of the magnetic layer on the underlayer, to micronize the crystal grain size of the magnetic layer, needless to say, the micronization of the gain size of the underlayer becomes an important technical subject.
Compared with the conventional medium, the medium of the present invention has a complex layer structure. Thus, the underlayer that has been described will be referred to as a non-magnetic intermediate layer hereinafter for convenience.
A first object of the invention is to provide a perpendicular magnetic recording medium having low noise and thermal stability by developing a non-magnetic intermediate layer capable of enhancing the perpendicular orientation of an easy magnetization axis with respect to a film surface, and simultaneously micronizing a magnetic crystal grain.
A second object of the invention is to provide a magnetic storage apparatus having a recording density of 50 gigabits or more per 1 square inch, by making sufficient use of performance of the above magnetic recording medium.
The first object of the invention is achieved by a perpendicular magnetic recording medium, comprising a non-magnetic intermediate layer, and a magnetic recording layer, which are sequentially formed. In this case, the non-magnetic intermediate layer has a face-centered cubic structure, and contains non-magnetic elements excluding Pt. Specifically, the intermediate layer is composed of a film mainly containing one selected from the group of elements constituted of Al, Cu, Rh, Pd, Ag, Ir and Au, and having a face-centered cubic (f. c. c.) structure. The magnetic recording layer is composed of a film containing at least Co, Cr and Pt, and having a hexagonal close-packed (h. c. p.) structure.
In the case of the film having the f. c. c. structure, if [111] is grown perpendicularly to a film surface, then (111) is grown within the film surface. On the other hand, in the case of the film having the h. c. p. structure, if [0001] is grown perpendicularly to a film surface, then (00.1) is grown within the film surface. As shown in FIG. 2, the atom arrays of the f. c. c. structure (111) and the h. c. p. structure (00.1) exhibit the same closest packing surfaces. When the process of each film crystal growth is closely observed atomic layer by layer, there is no difference between the f. c. c. structure and the h. c. p. structure from a first atomic layer (A surface) to a second atomic layer (B surface) grown thereon. On a third atomic surface grown on the second atomic layer, for the h. c. p. structure, atoms of the third atomic layer are arrayed to overlap those of the first atomic layer in the same position. In other words, as the array of atomic surfaces perpendicular to the h. c. p. [0001] direction, an A surface, a B surface, an A surface, a B surface, . . . , are repeated. On the other hand, the third atomic layer of the f. c. c. structure has a new array surface not overlapping the atom positions of the first atomic layer and the second atomic layer (C surface). A fourth atomic layer laminated on the third atomic layer has an atom array surface similar to that of the first atomic layer. Thus, as the array of atomic surfaces perpendicular to the f. c. c. [111] direction, an A surface, a B surface, a C surface, an A surface, a B surface, a C surface, . . . , are repeated. Apparently, only the foregoing difference is present between the h. c. p. structure having [0001] oriented perpendicularly to the film surface, and the f. c. c. structure having [111] oriented. By taking out only an optional layer, atom array surfaces having the same closest packing structures are formed.
Therefore, since the f. c. c. (111) and the h. c. p. (00.1) have the same closest packing surfaces, the h. c. p. (00.1) may be grown on the f. c. c. (111) in a heteroepitaxial manner. As elements having f. c. c, structures, there are Al, Cu, Rh, Pd, Ag, Ir, Pt and Au. If these elements are (111) oriented, then an easy magnetization axis of the magnetic recording layer can be oriented perpendicularly. If a film having an f. c. c. structure is formed by sputtering and the like, generally a closest packing surface is oriented in-plane. In other words, since (111) orientation is easily obtained, perpendicular orientation of the magnetic recording layer is also easily obtained.
Lattice matching between (111) of Al, Cu, Rh, Pd, Ag, Ir, Pt and Au and (00.1) of Co depends on a distance between nearest neighbor atoms. The distance between the nearest neighbor atoms is, as shown in FIG. 3, a distance between the center positions of the neighbor atoms. To strengthen orientation for setting (00.1) of the magnetic recording layer having the h. c. p. structure parallel to the film surface, it is important to discover an intermediate layer having an f. c. c. structure close to a distance between the nearest neighbor atoms of Co. Distances between the nearest neighbor atoms in the h. c. p. and f. c. c. structures are respectively set, as shown in FIG. 4, equal to a axial lengths (h. c. p.), and 2xc2x7a/2(f. c. c.). Table 1 shows 2xc2x7a/2 of an element having an f. c. c. structure, in which 2xc2x7a/2 of Cu takes a value 0.2556 nm, closest to an a axis (0.2506 nm) of Co. In an actual magnetic layer, one obtained by adding an element, e.g., Pt, to Co, is generally used. In this case, since a distance of the magnetic layer between nearest neighbor atoms is extended, the distance substantially coincides with that between the nearest neighbor atoms of Cu. Thus, among Al, Cu, Rh, Pd, Ag, Ir, Pt and Au, Cu is most preferable for enhancing orientation of the magnetic recording layer.
On the other hand, among Al, Cu, Rh, Pd, Ag, Ir, Pt and Au, Pt is not preferable because of its low compatibility with the soft magnetic layer. According to the examination of the inventors, it was verified that providing a non-magnetic intermediate layer having an f. c. c. structure, adjacently to the soft magnetic layer was important for enhancing orientation of the magnetic recording layer. Generally, the soft magnetic layer contains Co or Fe. It may be necessary to subject the medium to heat treatment in order to perform magnetic domain control for the soft magnetic layer. In this case, diffusion occurs in an interface between the soft magnetic layer and the Pt non-magnetic intermediate layer, generating high coercivity in the surface layer of the soft magnetic layer. This may be attributed to the formation of the same structure as Coxe2x80x94Pt or Fexe2x80x94Pt, well known as a permanent magnet material, in the interface. Even if no heat treatment is necessary for magnetic domain control of the soft magnetic layer, the formation of a Pt non-magnetic intermediate layer by sputtering causes mixing in the interface with the soft magnetic layer depending on energy of sputtering particles, which may generate high coercivity. The high coercivity of the soft magnetic layer is not preferable, because it increases medium noise. Further, the same phenomenon occurs when no soft magnetic layer is provided. On the non-magnetic intermediate layer having the f. c. c. structure, a non-magnetic h. c. p. intermediate layer, or a magnetic recording layer is directly formed, and Co is contained in any of the layers. Thus, diffusion occurs in the interface between Co contained in these layers and the Pt intermediate layer, causing various problems. As can be understood from the foregoing, Pt must be excluded though it has an f. c. c. structure, and it is important to select elements constituting the non-magnetic intermediate layer among Al, Cu, Rh, Pd, Ag, Ir and Au.
Depending on the kind of an element added to the magnetic recording layer, a distance between the nearest neighbor atoms of the magnetic recording layer may become extremely large. In such a case, it is necessary to select an element for the non-magnetic intermediate layer having the corresponding lattice size. In addition, rather than selecting an element only from the foregoing crystallographic viewpoint, it is important to select at least one from the group constituted of Al, Cu, Rh, Pd, Ag, Ir and Au from the viewpoint of chemical stability. A plurality of these elements may be selected, and alloyed. In such a case, the alloy must have an f. c. c. crystal structure.
The inventors examined the micronization of the crystal grain size for the Cu non-magnetic intermediate layer. For controlling the crystal grain size of a film formed by sputtering, a melting point of an element constituting the film is an important factor. As shown in Table 1, a Cu melting point is 1085xc2x0 C., which is not so high. Generally, in the case of a low melting point, because of high mobility after the sticking of sputtering particles to the substrate, the crystal grain size tends to be increased. The increase of the grain size in turn causes an increase in medium noise, and thus the crystal grain must be micronized to a certain extent. To micronize the crystal grain size of the Cu non-magnetic intermediate layer, preferably, at least one element selected from V, Cr, Nb, Mo, Ta and W should be added by an amount set in the range from 0.5 at. % to 30 at. %.
The melting points of V, Cr, Nb, Mo, Ta and W are respectively 1905xc2x0 C., 1875xc2x0 C., 2468xc2x0 C., 2615xc2x0 C., 2998xc2x0 C., and 3380xc2x0 C., which are all high compared with that of Cu. By adding these elements of relatively higher melting points by amounts set in the range from 0.5 at. % to 30 at. %, the increase of the Cu crystal grain size can be prevented. In addition, all of these added elements have body-centered cubic (b. c. c.) structures as single bodies, and each is a eutectic type when a binary alloy phase diagram with Cu is examined. Generally, the addition of an element as a two-phase separation system is advantageous for micronizing the crystal grain size, and may also be advantageous for micronization from the viewpoint of a phase diagram.
There is no limitation placed on a material for the soft magnetic layer of the medium of the invention. A principal point of the invention is to control the crystal orientation of the magnetic recording layer by providing the foregoing non-magnetic intermediate layer of Cu or the like on the soft magnetic layer, and this effect will not be varied by the material of the soft magnetic layer. In the examination of the invention, the soft magnetic layers of FeNi, CoTaZr, FeTaC, and so on, were used, and there was no variation in the effect of the non-magnetic intermediate layer of Cu or the like for the enhancement of the crystal orientation of the magnetic recording layer.
Japanese Patent Application Laid-Open Hei 6 (1994)-76260 discloses a technology, which uses a Cu non-magnetic intermediate layer. In this conventional art, a soft magnetic layer is provided between the Cu intermediate layer and a magnetic recording layer. The invention is largely different from the conventional art in that the soft magnetic layer is provided between the substrate and the Cu intermediate layer. According to the examination by the inventors, it was discovered that the direct lamination of the magnetic recording layer on the soft magnetic layer caused magnetic coupling between the two layers, consequently reducing the coercivity of the magnetic recording layer. Thus, the inventors concluded that it was important to provide a non-magnetic intermediate layer between the soft magnetic layer and the magnetic recording layer. In other words, the Cu non-magnetic intermediate layer has a function of controlling the crystal orientation of the magnetic recording layer, and an additional function of cutting off magnetic coupling between the soft magnetic layer and the magnetic recording layer. Therefore, the present invention cannot be inferred from Japanese Patent Application Laid-Open Hei 6 (1994)-76260. If the medium layer structure is different, then characteristics are totally different. Further, in Japanese Patent Application Laid-Open Hei 6 (1994)-76260, CoPtBO is used for a magnetic layer, and different from the invention, Cr is not contained. According to the result of the examination by the inventors, it is essential to add Cr in order to achieve high coercivity of the medium, and low noise. Therefore, this point is also different from the conventional art.
In order to achieve high coercivity of the medium and low noise, it is important that the magnetic recording layer mainly contains Co, and at least Cr by an amount of 15 at. % to 25 at. %, and Pt by an amount of 10 at. % to 20 at. %. In addition, when B, Ti, Nb and Ta are added to achieve low noise, it is important to set the total concentration of these elements equal to 8 at. % or lower to prevent the non-magnetization of the magnetic layer. In the magnetic layer composition, at least Co need be set equal to 62 at. % or more. If Co concentration drops below 62 at. %, a conspicuous reduction occurs in a residual magnetic flux density, reducing magnetic flux leaked from the medium, and consequently making it difficult to read a signal by the magnetic head.
Preferably, a non-magnetic h. c. p. intermediate layer containing at least Co and Cr, and having the added concentration of Cr set in the range from 28 at. % to 45 at. % should be provided between the non-magnetic f. c. c. intermediate layer of Cu or the like and the magnetic recording layer, in order to increase coercivity and squareness. The increase of squareness provides an effect for suppressing a reduction with time in read output caused by the influence of thermal fluctuation. The (111) of non-magnetic f. c. c. intermediate layer of Cu or the like, and the (00.1) of the magnetic recording layer are atom arrays having the same closest packing structures. However, because of difference in kind between f. c. c. and h. c. p. crystal structures, slight disturbance occurs in heteroepitaxial growth in an interface thereof. Therefore, by providing a second non-magnetic intermediate layer having the same h. c. p. structure as that of the magnetic recording layer, the crystal orientation of the magnetic recording layer can be enhanced. When a non-magnetic intermediate layer containing Co and Cr is used, since the intermediate layer is non-magnetized maintaining the h. c. p. structure, the added concentration of Cr need be set in the range from 28 at. % to 45 at. %.
A layer thickness of the non-magnetic intermediate layer need be set in the range from 0.3 nm to 25 nm. In the case of the double-layered structure of the non-magnetic f. c. c. intermediate layer of Cu or the like and the non-magnetic h. c. p. intermediate layer, a total layer thickness need be set within the above range. If a layer thickness is smaller than 0.3 nm, the intermediate layer itself cannot be formed as a stable crystal film. In addition, it is not preferable because of difficult production management or the like. On the other hand, if a layer thickness exceeds 25 nm, a spacing loss between the soft magnetic layer and the magnetic recording layer is increased, and a recording magnetic field becomes steep, making it impossible to narrow the width of a magnetization transition region formed in the medium. In other words, high-density recording cannot be realized. One of the significant points of providing the soft magnetic layer is to draw in and make steep a recording magnetic field, and in order to achieve the object of the invention, a total thickness of the intermediate layer need be set equal to 25 nm or lower.
The second object of the invention is achieved by a magnetic storage apparatus, comprising: a magnetic recording medium; a driver for driving the magnetic recording medium in a recording direction; a magnetic head composed of recording and reading units; means for moving the magnetic head in relative relation to the magnetic recording medium; and read/write signal processing means for subjecting an input signal to the magnetic head, alternatively an output signal from the same, to wave form processing. In this case, as the recording medium, the foregoing perpendicular magnetic recording medium of the above-described invention is used. By the magnetic storage apparatus of the invention, a recording density of 50 gigabits or more per 1 square inch can be achieved. As the perpendicular magnetic recording medium, according to the invention, a single-layered perpendicular medium having no soft magnetic layer can be used, and as the magnetic head, one including a recording unit composed of a ring head can be used. Alternatively, as the perpendicular magnetic recording medium, according to the invention, a double-layered perpendicular medium having a soft magnetic layer can be used, and as the magnetic head, one including a recording unit composed of a single magnetic pole head can be used. Preferably, the reading unit of the magnetic head should be composed of a magnetoresistive device.